1. Field of Invention
The invention pertains generally to ultrasonic imaging systems and is more particularly directed to use of such systems for nondestructive testing of parts having inaccessible internal surfaces, or for parts having suspected internal flaws determinable only by access to external surfaces.
2. Description of Related Art
Nondestructive testing (NDT) relates to that science which can be used to gain information about the integrity of structures by noninvasive techniques. These systems are useful in the manufacturing and construction processes to control the quality of the work product as well as in maintenance programs to test the integrity of a workpiece in use. A typical application where NDT might be used is in the examination of welds for internal flaws or measuring thicknesses of precision parts. Aircraft, submarines and nuclear waste tanks are only a few of the areas where NDT interrogation has unquestioned utility.
The underlying principle upon which these systems are founded is a basic physics relationship: distance=velocity.times.time. Innocuous sound waves are used to determined the desired distance in the measured part. The velocity of sound in the material is a constant and is typically found through use of a reference book (e.g. The CRC Physics Handbook) or determined experimentally using a known good part of precision thickness. Such determinations are well known to those skilled in the art.
The time variable depends upon the thickness of the part or upon the location of the defect in the part. As sound waves travel through the part, a portion of its energy will be reflected at the interface of a discontinuity having a different refractive index. The period of time from initial transmission of the wave until the reflected energy has been detected is directly proportional to the thickness of the part, or similarly, to the location, shape, size and orientation of a defect (e.g. an air pocket) in the part.
Typically, measurements are accomplished by emitting a pulse from an ultrasonic transducer located directly on the surface of the part to be tested. Often a transmissive couplant gel is used to couple the ultrasonic pulse to the surface. Simultaneously with the generation of an ultrasonic pulse into the material to be tested, the imaging system starts a clock or counter. As, the transducer receives reflections or "echoes", the imaging system compares the level of the resultant signal to a preset threshold value. When the threshold is crossed the clock(s) or counter(s) is stopped to indicate the elapsed time T to and from a "discontinuity". Distance is then calculated by use of the equation D=V.times.T/2.
In the case of using a clock to measure time directly, high accuracy can be obtained depending on the clock's oscillating frequency. For example a clock with a 1 GHz oscillating frequency would provide a very accurate time reading for most purposes. However, for many applications such a high frequency clock would be prohibitively expensive. It would be desirable to be able to measure time using a less expensive timing means operating at a lower frequency without incurring a substantial reduction in reading accuracy.
A major problem with using counters to determine elapsed time is that a user is never really quite sure what turned the timer off. In the prior art, a voltage threshold is employed and set to trigger when the amplitude of the reflected waveform exceeds the threshold value. (A time delay may be added to ignore possible triggering due to a reflection from the pulse's initial entry into the material.) However, it is difficult for a user to be certain that a bona fide defect has triggered cessation of the counter rather than spurious noise. Often, this uncertainty requires extensive retesting of the suspect areas.
Also, in the case of a defect, the amplitude of the reflected energy is often related to the shape, orientation and physical size of the discontinuity. As such, the user might want to know by how much the amplitude of the returning signal exceeded her preset threshold value for purposes of quality control. Multiple counters often are employed and set at various thresholds and turned on at various times of interest which require more expensive and complex circuits.
The instant invention seeks to eliminate many of these difficulties. It is an object of this invention to provide an accurate time measurement and allow the user to quickly and easily distinguish a true defect signal from random noise.
It is another object of this invention to allow the user to examine various amplitude threshold values without the need for multiple counters or redundant testing of a location.
It is a further object of this invention to provide a relatively inexpensive and low power means of measuring time that does not substantially sacrifice reading accuracy.